In lithographic printing plates, oleophilic image areas materially corresponding to an image and hydrophilic non-image areas are formed in the same plane. The oleophilic image area is ink-receptive and water-repellent, the hydrophilic non-image area is ink-repellent and water-receptive. In the printing process, the ink that is adhered to the image areas is transferred to the printing-paper via a rubber blanket, whereas the non-image areas are wetted with water and do not transfer any ink to the paper. Lithographic printing plates are formed by a light-sensitive layer that is coated on a suitable support. The photosensitive layers can be divided into two types:
Positive working layer, in which the unexposed area remains on the plate after development and thus forms the image area PA1 Negative working layer, in which the unexposed area is removed in the development process to reveal the non-image area. PA1 (a) an organic solvent PA1 (b) a surfactant PA1 (c) a gelatine, which comprises more than 3% of the PA1 preferably from 4%b/w to 6%b/w on the basis of the total weight of developers used.
Developer compositions are usually water-based systems, because of environmental and safety reasons. In many cases they contain one or more surfactants and an (low amount of) organic solvent. In the U.S. Pat. No. 4,186,006 , benzyl alcohol and the sodium salt of t-Butyl-naphtalene sulphonic acid are described for usage in developer as solvent and surfactant respectively.
Because of the nature of the lithographic plate, binders are usually hydrophobic of nature, which poses the problem that they do not readily dissolve in aqueous systems. To overcome this problem, it is possible to incorporate into the binder some acidic side-groups, which yield a water soluble compound when treated with an alkaline liquid. Examples of these systems can be found in U.S. Pat. Nos. 4,631,245 and 4,902,602. These compounds however tend to have a lower sensitivity and less ink receptivity than binders without acidic side-groups. The less materials groups will result in a higher sensitivity and better ink receptivity, but a poorer solubility in a developer.
Binders that do not dissolve into the developer liquid sufficiently, will form coagulates: they will stick together, forming large, insoluble particles. These particles have an affinity for parts in the automatic development machine that have a hydrophobic surface: brushes, rubber rollers, tank walls. The insoluble particles will adhere to these parts, accumulate, and in time will redeposit on the non-image area of the printing plate, yielding undesired image formation. It costs a lot of effort to clean the developing machine. So higher frequency of and longer time for cleaning will be necessary when that problem remains.
In EP 0004014, a developing composition is described consisting of a water soluble polymer, an anionic surfactant and a water miscible organic solvent, which is characterized by the water soluble polymer being either poly-N-vinyl-N-methylacetamide or polyvinylalcohol. It is further suggested to add 0.01-2 wt % of gelatin. Also in EP 323836, addition of natural polymers including gelatin is mentioned as a thickener. But the effect thereof is insufficient to prevent the problems discussed above.